10 years ago
They have a large number of Czech CO2 pistols that were made about 20 or
more years ago. They shoot very accurately, and are generally pretty
reliable. The primary firing seal is dying on a few of them, and the
replacements we have don't fit due to a design change at some point.
The seal is a short hard white plastic tube, press fit into a hole in the
pistol frame. The actual firing seal is formed with a conical pin that
presses into the hole through the center of the plastic seal. On the
theory that the most likely seal material is Nylon 6/6, I bought some rod
stock, and I'm going to try making new seals from scratch.
The recommended approach for removing the old seals it to run a brass
wood screw into the seal & then pull it out. The pulling part takes
quite a bit of force, and the process means the seal can't be re-used
once removed. However, the one I extracted is sufficiently intact to
take some measurements from. Give or take a few tenths, the seal
measures 0.3157" OD. The bore in the pistol measures 0.3138" ID.
I've done some reading on both Nylon & press fitting plastics, and there
are several issues that complicate matters:
1) Nylon is hygroscopic, and will swell depending on the moisture
content. That means if I machine one now when it is humid, it will
shrink in the winter. On the otherhand, machining it will heat it a bit
& dry it out slightly.
2) Nylon has a pretty high thermal coefficient of expansion. That means
I probably need to let it cool off to measure it when I'm machining it.
They recommend chilling the part to shrink it for the press fitting, but
with high humidity, if I get it at all cool, it will be covered with
moisture (if not frost). I'd have to drop the temperature ~ 60 degrees F
to shrink the part by 1 mil, which definitely gets it into the frost
3) If I make it sufficiently oversized to make sure that moisture &
thermal variations don't allow it to shrink enough to be a problem, it
can creep over time & loosen up. I suspect that may be why some of them
are failing after all this time, possibly shrinking enough to allow CO2
to sneak past them. There's a good web site that talks about this here:
This also means that the measured size of the seal I removed is probably
smaller than it was originally cut. As extracted, they had about a 2 mil
interference fit, which would seem to be plenty. I need to double check
the calculations, but that should result in over 1500 psi of expansive
force between the seal & the bore of the hole. It's also enough to
produce significant creep over time:
1% is over 3 mils, although as it creeps, the forces will drop, and
presumably it will reach something like steady state when the force drops
4) Annealing: Lots of folks recommend annealing Nylon before trying to
machine it. From some DuPont literature, 3/8" rod stock should only
require an hour at a couple hundred F. They recommend not doing it in
air, and suggest heating it in wax or oil, and then cooling it slowly &
evenly to avoid surface stresses. I'm thinking heating it in cooking oil
with a thermometer, and then letting the whole mess cool slowly.
There are other plastics that have lower moisture issues than Nylon 6/6,
but I think the TCE & creep is going to be similar for a lot of them.
I seem to have opened a serious can of worms here. I'm thinking about
just going for it, cutting one 2.5 mils oversized without annealing. If
anyone has any real experience to help sort through the chaos, I'm all